While you are fasting, there isn’t any glucose readily available, so your body dips into its glycogen stores for energy.
Once your glycogen becomes depleted, your body will then turn to one final source of energy, fat. This is how intermittent fasting works.
Intermittent Fasting Theoretically Resembles How Our Ancestors Ate
Before the agricultural revolution, our ancestors had to produce or hunt their own food.
Theoretically,there were probably long stretches of time where food wasn’t readily available to them, and they had to go through periods of fasting by default.